Australian Citizen Allegedly Pictured on Twitter Holding Two Severed Heads

CANBERRA, Australia—Australian police Tuesday issued arrest warrants for two Australian citizens the government accuses of fighting for the Islamic State of Iraq and al-Sham and photographing the severed heads of Syrian soldiers.

The Australian government is increasingly concerned about extremists returning and mounting attacks on home soil. Australia has become the largest per capita source of foreign fighters within ISIS ranks, with as many as 150 citizens and residents thought to have left the country to join militant...